There are two kinds of generally known methods of pumping bulk material through a pipeline which leads to a remote pouring position. One is a screw pumping method whereby bulk material can be pumped into a transportation conduit by means of a motor-driven feeder screw or auger. This method has been extensively used in pumping a uniform amount of low viscosity bulk material with reduced pulsation but has a drawback in that it is unsuitable for use in high-pressure, long-distance pumping of the bulk material because the drive motor cannot endure severe overload condition encountered.
The other method of pumping bulk material is to use a pair of hydraulic drive cylinders which are designed to cause alternate reciprocating movement of dual pump cylinders to thereby force the bulk material into a transportation pipeline. The drive cylinder type pumping method can be advantageously utilized in high-pressure, long-distance transportation of high density and high viscosity bulk material such as concrete, mortar and industrial wastes, although it tends to produce a great deal of vibration and pulsation in the pumping process.
With reference to FIGS. 1 through 3, there is illustrated a prior art drive cylinder type concrete pump device widely employed in a typical concrete pump truck. The concrete pump device has a reversible oil pump 10 which can discharge a variable volume of pressurized oil, and first and second hydraulic drive cylinders 12, 14 each of which remains in fluid communication with the oil pump 10. The first drive cylinder 12 consists of a cylinder housing 16 with an axial bore 18 and a drive piston 20 with a piston rod 22 slidably fitted into the cylinder housing 16 to divide the axial bore 18 of the cylinder housing 16 into a base chamber 24 and a head chamber 26. The volume of the base and head chambers 24, 26 varies with the position of the drive piston 20 in such a manner that, if the volume of the base chamber 24 becomes greater, that of the head chamber 26 gets smaller in proportion thereto and vice versa. The base chamber 24 is connected to the oil pump 10 via a first fluid line 28 which serves both as a supply line and a return line depending on the direction of rotation of the oil pump 10.
The second drive cylinder 14 consists of a cylinder housing 30 with an axial bore 32 and a drive piston 34 with a piston rod 36 slidably received in the cylinder housing 30 to divide the axial bore 32 of the cylinder housing 30 into a base chamber 38 and a head chamber 40. As with the first drive cylinder 12 set forth above, the volume of the base and head chambers 38, 40 in the second drive cylinder 14 varies with the position of the drive piston 34 in such a manner that the volume of the head chamber 40 becomes smaller in proportion to the increase of the volume of the base chamber 38 and vice versa. The base chamber 38 is coupled to the oil pump 10 by way of a second fluid line 42 which functions both as a supply line and a return line. The head chamber 40 of the cylinder housing 30 of the second drive cylinder 14 is in fluid communication with the head chamber 26 of the cylinder housing 16 of the first drive cylinder 12 via an intermediate fluid line 44.
The concrete pump device further includes first and second pump cylinders 46, 48 operatively connected to the first and second hydraulic drive cylinders 12, 14. The first pump cylinder 46 has a pumping barrel 50 with an open front end and a pumping piston 52 slidably received in the pumping barrel 50. The pumping piston 52 is affixed to the piston rod 22 of the first drive cylinder 12 so that it can be subjected to reciprocating movement together with the drive piston 20 of the first drive cylinder 12, thus pumping the concrete contained in a hopper 54. The second pump cylinder 48 has a pumping barrel 56 with an open front end and a pumping piston 58 slidably fitted through the pumping barrel 56. The pumping piston 58 of the second pump cylinder 48 is secured to the piston rod 36 of the second drive cylinder 14, meaning that the pumping piston 58 can move together with the drive piston 34 of the second drive cylinder 14 to thereby pump the concrete contained in the hopper 54. A transportation conduit 60 is alternately coupled to the respective one of the pumping barrels 50, 56 just prior to the extending movement of the pumping pistons 52, 58, thus receiving the concrete pumped by the first and second pump cylinders 46, 48. Alternate coupling of the transportation conduit 60 to the first and second pump cylinders 46, 48 is performed by a hydraulic switching actuator not shown in the drawings.
It will be noted that a head bypass line 62 is provided at the head portion of the cylinder housing 16 to allow fluid introduction from the base chamber 24 into the head chamber 26 when the drive piston 20 of the first drive cylinder 12 is at the end of retracting movement as indicated in a phantom line in FIG. 1. Such fluid introduction through the head bypass line 62 helps increase the pressure in the head chamber, thus swiftly reducing the retracting speed of the drive piston 20 and hence avoiding any crash of the piston 20 against the head cover of the cylinder housing 16. Backflow of fluid through the head bypass line 62 is inhibited by a check valve 64 even though the pressure in the head chamber 26 becomes higher than the pressure in the base chamber 24 at the beginning of extending movement of the drive piston 20.
Provided at the base portion of the cylinder housing 16 of the first drive cylinder 12 is a base bypass line 66 which permits fluid introduction from the head chamber 26 into the base chamber 24 when the drive piston 20 of the first drive cylinder 12 is at the end of extending movement as illustrated in a solid line in FIG. 1. Such fluid introduction into the base chamber 24 through the base bypass line 66 helps increase the pressure in the base chamber 24, thereby quickly decreasing the extending speed of the drive piston 20 and hence avoiding any crash of the piston 20 against the base cover of the cylinder housing 16. Backflow of fluid through the base bypass line 66 is prohibited by a check valve 68 even though the pressure in the base chamber 24 grows higher than the pressure in the head chamber 26 at the beginning of retracting movement of the drive piston 20.
Likewise, a base bypass line 70 is provided at the base portion of the cylinder housing 30 of the second drive cylinder 14. The base bypass line 70 allows fluid introduction from the head chamber 40 into the base chamber 38 when the drive piston 34 of the second drive cylinder 14 is at the end of extending movement as shown in a phantom line in FIG. 1. Such fluid introduction through the base bypass line 70 helps increase the pressure in the base chamber 38 to thereby reduce the extending speed of the drive piston 34 for avoidance of its crash against the base cover of the cylinder housing 30. Backflow of fluid through the base bypass line 70 is prevented by a check valve 72 even though the pressure in the base chamber 38 becomes greater than the pressure in the head chamber 40 at the beginning of retracting movement of the drive piston 34 of the second drive cylinder 14.
A retraction sensor 74 and an extension sensor 76 are placed respectively at the head portion and the base portion of the cylinder housing 30 of the second drive cylinder 14. The retraction sensor 74 is adapted to issue a piston retraction signal as the drive piston 34 moves past the retraction sensor 74 at the end of retracting movement thereof. Similarly, the extension sensor 76 serves to generate a piston extension signal as the drive piston 34 moves past the extension sensor 76 at the end of extending movement. The piston retraction signal and the piston extension signal so produced are fed to a pump controller 78 which in turn will change the direction of rotation of the oil pump 10 each time one of the piston retraction and extension signals are received. This enables the drive piston 20 of the first drive cylinder 12 and the drive piston 34 of the second drive cylinder 14 to move in the reverse direction, causing alternate reciprocating movement of the pumping pistons 52, 58 of the first and second pump cylinders 46, 48'.
According to the prior art concrete pump device explained above by way of example, if the oil pump 10 feeds pressurized oil through the first fluid line 28, the drive piston 20 of the first drive cylinder 12 will be retracted together with the pumping piston 52 to suck in the concrete from the hopper 54 and, at the same time, the drive piston 34 of the second drive cylinder 14 will be extended together with the pumping piston 58 to discharge the concrete into the transportation conduit 60. During the course of such a "loaded" operation, the oil pressure will be greatest in the base chamber 24, medium level in the head chambers 26, 40 and lowest in the base chamber 38. Thus the oil in the base chamber 24 will be admitted into the head chamber 26 through the head bypass line 62 at the end of the retracting movement of the drive piston 20 to thereby suppress further retracting movement thereof, while the oil in the head chamber 40 of the second drive cylinder 14 will be introduced into the base chamber 38 via the base bypass line 70 at the end of extending movement of the drive piston 34 to thereby retard further extending movement of the latter.
In the event that the oil pump 10 is rotated in the reverse direction to feed pressurized oil through the second fluid line 42 to cause retracting movement of the drive piston 34 and extending movement of the drive piston 20, the oil pressure will be greatest in the base chamber 38, medium level in the head chambers 40, 26 and smallest in the base chamber 24. As a result, the oil in the head chamber 26 will be admitted into the base chamber 24 through the base bypass line 66 at the end of extending movement of the drive piston 20 to thereby retard further extending movement of the latter. It can be seen from the foregoing that, in the course of loaded operation of the concrete pump device, almost the same amount of the oil introduced into the head chamber 26 through the head bypass line 62 at the end of retracting movement of the drive piston 20 is returned back to the base chamber 24 through the base bypass line 66 at the end of extending movement of the drive piston 20. This means that no surplus oil is accumulated in the head chamber 26 during the loaded operation of the concrete pump device.
Such is not the case in case of load-free idle operation of the concrete pump device with the hopper 54 being empty. Specifically, if the pressurized oil is fed through the first fluid line 28 to cause retracting movement of the drive piston 20 of the first drive cylinder 12, as shown in FIG. 2, the oil pressure(typically 26 bar) in the head chambers 26, 40 is kept lower than the oil pressure(35 bar) in the base chamber 38 of the second drive cylinder 14 as well as the oil pressure(45 bar) in the base chamber 24 of the first drive cylinder 12. Accordingly, the oil in the base chamber 24 will be introduced into the head chamber 26 through the head bypass line 62 at the end of rightward, retracting movement of the drive piston 20, as in the loaded operation described above.
However, the oil pressure in the base chamber 24 will not drop to below the pressure in the head chamber 26 during the extending movement of the drive piston 20 under the load-free, idle operation condition, as shown in FIG. 3. This means that the oil introduced into the head chamber 26 through the head bypass line 62 during the end of the extending movement cannot be returned back to the base chamber 24 even at the end of the leftward, extending movement of the drive piston 20 and therefore will be accumulated in the head chamber 26. Each time the drive piston 20 is subjected to one cycle of reciprocating movement, therefore, the stroke of the drive piston 20 is shifted toward the base cover of the cylinder housing 16 by the displacement ".chi." (see FIG. 3) which corresponds to the amount of the oil introduced into the head chamber 26 during the retracting movement of the drive piston 20. Repeated reciprocating movement of the drive piston 20 in this manner under the idle operation condition will result in gradual increase of the stroke shifting amount, eventually causing the drive piston 20 to make crash against the base cover of the cylinder housing 16. Such crash is a major culprit in producing noise and adversely affect the structural integrity of the concrete pump device, which may lead to damage and shortened service life of key parts of the concrete pump device.